Complex radio signals can be used for digital audio communications, as is well known to those of ordinary skill in the art. For example, one common complex transmission scheme is orthogonal frequency-division multiplexing (OFDM), which can be used for terrestrial digital video broadcasting (DVB-T), digital audio broadcasting (DAB-T), wireless local area networks, and wireless metropolitan area networks. However, the well-known advantages of OFDM are somewhat counter-balanced by a significant side-effect: high peak-to-average power ratio. High peak-to-average power ratios of OFDM significantly reduces the average power at the output of the high-power amplifier (HPA) used at the transmitter. Relatively expensive HPA's are required because, as is true of many power amplifiers, the input signal must lie within the linear range of the power amplifier, and because there is an increased linear dynamic range, the power amplifiers require expensive components and more complicated designs to provide such dynamic range. Some solutions to this problem include digital reduction of the peak-to-average power ratios.
Digital reduction of the peak-to-average power ratio (PAPR) of a complex radio signal, while filtering the signal to remove out of band emissions, provides a way to reduce the peak demand on a power amplifier. Digital reduction of the PAPR of a complex radio signal enables improved efficiency and reduces the cost of a cellular radio system with a small, but acceptable degradation in signal quality or error vector magnitude (EVM). From here-on in, throughout this discussion, the term “peak power reduction” (PPR) shall be used in place of PAPR for convenience.
Orthogonal frequency division multiplexing (OFDM) technologies have complex radio signals with many closely spaced sub-carriers, each of which can have a very different EVM requirement. However, the close frequency spacing increases the difficulty in providing differentiation from an EVM perspective while reducing the PAPR. Existing technologies do not efficiently differentiate between sub-carriers and the required reserved sub-carriers. In addition, there are existing time domain based technologies, such as wideband code division multiplexing multiple access (WCDMA), code division multiple access (CDMA), global system for mobile communications (GSM) and others, that often continue to be used in the same band and sharing the same transmitter where OFDM signals are being used. Thus, the peak power reduction problem becomes even more pronounced and difficult to solve. In OFDM, it is known that there can be, for example up to 600 in-band sub-carriers and up to 424 guard band sub-carriers, for a total of 1024 “sub-carriers” in each band, or “carrier”. As those of ordinary skill in the art understand, each sub-carrier in an OFDM communications system is an integer multiple of a base sub-carrier frequency, which is what makes each sub-carrier orthogonal to each other, and hence non-interfering. Accordingly, OFDM communications systems have been implemented wherein the sub-carrier frequency spacing is about 15 KHz.
With the use of OFDM technologies, the signal is composed of an array of sub-carriers of varying tolerance to degradation in signal quality. That is, different sub-carriers have different EVM requirements. Some of the existing time domain solutions are unable to make a distinction between the sub-carriers, and so time domain solutions are limited in the amount of PPR that can be applied to the least tolerant sub-carrier (of an OFDM signal). In other cases, the existing solutions do not differentiate between traffic sub-carriers and required reserved tones (or reserved sub-carriers) that take away from the allocated spectrum for normal communication (See, for example, “Apparatuses and a Method for Reducing Peak Power in a Transmitter of Telecommunications Systems”, U.S. Published Patent Application No. 2009/0176466 A1, published on Jul. 9, 2009, by Richard Hellberg, and Torbjorn Widhe, the entire contents of which are incorporated herein by reference; and “System and Method for Reducing Peak-to-Average Power Ratio in Orthogonal Frequency Division Multiplexing Signals Using Reserved Spectrum”, U.S. Pat. No. 7,583,583, Issued Sep. 1, 2009, to Ning Guo, Neil McGowan, and Gary Boudreau, the entire contents of which are incorporated herein by reference). Other existing solutions differentiate between traffic sub-carriers, but still require reserved sub-carriers that take away from the allocated spectrum for normal communication (See, for example, “Method and System for Adaptive Peak-To-Average Power Ratio Reduction in Orthogonal Frequency Division Multiplexing Communication Networks”, U.S. Published Patent Application No. 2009/0092195 A1, published on Apr. 9, 2009, by Ning Guo, Neil McGowan, and Bradley John Morris, the entire contents of which are incorporated herein by reference). Furthermore, all of these prior methods produce and pass between iterations a time domain representation of the signal. This is problematic for at least two reasons: first, there can be problems differentiating between sub-carriers; and second, a conversion must take place. When OFDM and time domain technologies are used sharing the same band and transmitter, then existing PPR systems and methods use a time domain based system that have all of the above limitations.
Consequently, because of the problems associated with performing PPR for both OFDM and time domain signals through time domain solutions alone, as briefly discussed above, it would be desirable to provide methods, modes, and systems for performing peak power reductions using other than just the time domain solutions previously proscribed that will obviate or minimize the problems associated with prior art solutions. Performing PPR in the frequency domain together with a phase slope provides the use of low-sample rates for multiple carriers in the frequency domain and use of guard bands for PPR.